The follower is facing away from you. Attack it will cause up to two more to drop down. You will also be introduced to floor mines. Shoot them, and they will explode. They will not react to enemies, only you. Proceed through the tunnel, to reach a small canyon. Jump down, head to the right, fight the enemies, and flip the switch to open the bars blocking the north. Proceed past these open bars, and fight the two Hunters and Phage. Collect the green key.